Remote plasma sources (RPS) are used to produce activated gases containing ions, free radicals, atoms and molecules by passing a gas through a plasma that excites the gas. Activated gases and free radicals are used for numerous industrial and scientific applications including processing solid materials and/or thin films such as those found on semiconductor wafers, display panels, and other active device substrates. Activated or energized gases containing ions, dissociated atoms and free radicals are also used to remove deposited thin films from semiconductor processing chamber walls. One additional use of an RPS is to ionize and reduce the gaseous waste in a pump stream such that pumping is expedited by the plasma temperature and compound gases can be broken down to safer elemental form by the plasma ionization of the stream.
An RPS has been developed in which gases are introduced into a linear inductive plasma chamber, and the gases are dissociated into reactive species that flow out of the chamber and are used to perform work (e.g., on substrates) downstream of the source. In these types of RPS, an inductive coil is typically disposed axially about the chamber to inductively induce currents within the plasma.
Toroidal sources have also been developed in which currents are induced in a toroidal plasma contained within a general toroidal chamber. These Toroidal sources often use a ferrite based transformer arrangement to inductively couple fields produced by a primary set of windings, concentrate the fields in the ferrite core and then induce currents in the plasma which make up the secondary loop of the transformer.
These inductive linear and inductive toroidal remote sources have several drawbacks. For example, they typically only operate optimally when the plasma is in a high current state, which is referred to as an ICP state or sometimes as H-mode operation. This state typically cannot be produced without reaching a critical threshold of field strength and current density within the source. Consequently, low power, high pressure operation can be difficult and often not possible. Similarly, operation in the ICP regime requires adequate charge density in the plasma to be reached, and achieving this state during plasma ignition can be difficult. For this reason, these sources often require ignition to be accomplished using an easy-to-ionize gas, such as argon, before the processing gas can be introduced.
In addition, toroidal sources are traditionally designed from electrically and thermally conductive materials such as aluminum, which requires water cooling due to the high power densities that are required to operate these sources. To prevent current from flowing through the chamber body instead of through the plasma, the design of toroidal sources typically incorporates dielectric breaks in the body. These dielectric breaks experience a high voltage drop (i.e. a high electric field) in their proximity that results in ion acceleration to the walls of the source. This ion acceleration results in erosion of the wall in these locations resulting in damage to the inner walls and shortened operating lifetime of the device. This ionic bombardment of the wall also results in particle generation. Particle generation in these sources can be particularly damaging to thin film structures, semiconductor devices or to the downstream tools being cleaned by these remote sources.
Linear inductive sources see non-uniform field distribution since the field strength is greatest close to where the coils wrap around the linear source. The result is that portions of the inside of the source etch faster via ion bombardment and thus linear sources often have shortened lifetimes.
As seen, existing RPS sources are often unsatisfactory and improved RPS sources will almost certainly be needed in the future.